Modulation system



Jan. 27, 1948. E. LABxN'E-r AL MODULATION SYSTEM Filed Aug. 24, 1942 2 SheetEs-Sheei'l l ou MV1 ummm- INVENTRS EMLE LAB/N DNALO D.

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Jan. 27, `1948. E. LABIN ETAI.

MODULATION SYSTEM n 2 Sheets-Sheet 2 Filed Aug.

INVENTORS EM/LE LAB/N BYDNALD D. R/E

ATT RNEY Patented Jan. 27, 1948 2,434,936 MoDULATioN SYSTEM Emile Labin, New York, and Donald D. Grieg, Y Forest Hills, N. Y., assignors to Federal Telephone and Radio Corporation, a corporation of Delaware ApplicationrAugust 24, 1942, Serial No. 455,898 9 Claims. (Cl. 179-1715) 1 The present invention relates to pulse generation systems and more particularly to systems for generating time modulated pulses for use in communication systems and for other uses. n

Several types of systems have been proposed for effecting communication by transmitting a number of pulses of current, the exact times of transmission of such pulses being controlled in accordance with the speech or other intelligence to be transmitted. In s-ome cases the pulses of current are themselves transmitted yto a distance over wires, but in other cases they are used to modulate carriers so as to produce brief wave trains generally referred to as carrier pulses. Such systems of communication have been described in U. S. Patents 2,266,401, 2,256,336, 2,265,337, 2,262,838, and in copending U. S. applications 386,282 and 425,108, respectively filed on April 1, 1941, by E. Labin and on December 31, 1941, by E. Deloraine and E. Labin, as well as in a large number of other foreign patents and applications. In accordance with the systems heretofore proposed, the generation of time modulated pulses was eiected either by means of special vacuum tubes having cam-like target or masking means cut to a predetermined shape, or by providing multivib-rator arrangements controlled by resistance/condenser time constants and capable of being influenced by the modulating signals so as to vary the vibration rate.

vide a simpler, more reliable, and more economical method of generating time modulated pulses.

More particularly, it is an object to provide a system for generating time modulated pulses by the use of ordinary types of circuit components without requiring specially designed tubes.

, It is a further object to provide such a system for generating time modulated pulses which shall be capable of being stabilized by a master source of iixed waves, such as a sine wave oscillator` or a stable unmodulated relaxation oscillator.

More particularly, it is an object to provide such a system wherein the time of the generated pulses is controlled by a fixed unmodulated wave.

Other objects of the invention will readily appear to those skilled in the art from the following detailed description of the invention, taken together with the annexed drawings, in which,

Fig. l is a schematic diagram of a time modulation transmitter embodying one form of our invention;

Figs. 2 and 3 are sets of curves used for explaining the operation of Fig. 1;

^ It is an object of the present invention to prov Fig. 4 is a schematic representation of an arrangement for producing triangular waves which may be substituted for a portion of the system of Fig. 1;

Fig. 5 is a schematic representation of a clipping circuit which may be substituted for the clipping circuit shown in Fig. 1;

Fig. 6 is a schematic diagram of a time modulation transmitter embodying a modied form of our invention; and

Fig. 7 is a set of curves used in explaining the operation of the system of Fig. 6.

Referring more particularly to Fig. 1, I0 is a saw-tooth wave generator synchronized by a sine lwave oscillator II and connected to deliver its saw-tooth output via a preliminary clipping amplier I2 to the main clipping circuit I3.

As shown in Fig. 1, this main clipping circuit I3 comprises a conventional pentode I4 connected in the usual resistance-coupled fashion but preferably having a high resistance I5 in series with its control grid lead and having a shorting-type jack I6 connected in the control grid return lead. Except for this high resistor l5 and the jack I6, the circuit connections of the pentode Ill are essentially conventional, comprising on the input side the usual coupling condenser I9, and the usual grid bias resistorV 20, the latter being connected through jack I6 to the usual C battery 2l. The output side of the circuit comprises the usual load resistor 22 connected to the usual plate supply battery 23, as well as a conventional output coupling condenser 24. The screen is biasedpositively by screen rbattery 25 in the usual manner. Preferably, the screen voltage battery 25 (and if desired the plate battery 23, also) is of lower potential than usual so that only a small negative control grid voltage is required for producing cut-off. The C bias battery 2| may, for convenience, be assumed to provide just suilcient potential to produce cut-01T.

In addition to the input applied from sawtooth generator Ill via amplier I2, an additional lower frequency modulatin: signal is applied to the circuit I3 through the jack I6. As illustrated in Fig. 1, the modulating input consists of speech from a telephone transmitter 3B which is applied via amplifier 3|, transformer 32, and plug 33 to jack I6 of the main clipping circuit I3, being thus applied to the control grid of pentode I4 in series with battery 3ft.Y

The output from the main clipping circuit I`3is fed through a wave differentiation network 35 which may be of any known type and thence through a rectifying and limiting circuit 3B, which may also serve for amplification if desired. The output obtained from circuit. 36 is then applied to modulate a radio frequency amplifier 38, excited from a suitable radio frequency source 31, 5 so as to produce a train of carrier Waves responsive to each modulating impulse [received'sfroml the circuitfl.; The R'.F.'output ofiamplier 38. is then applied to antenna 39 for radiation to a distant point.

The operation can best be understodhycon; sideration of the curves of Figs. 2 and 3 incon nection with the circuit of 'Fig. 1.? Thegsavvl tooth generator I0 produces Wavesisuchas shown amplier l2 then cuts oil the bottom portions of these waves so as to pass onlya wave `sucllgas: g shown in curve V of Fig, 3, commonly called a e blanked saw-tooth wave. Such wave lVistheiii applied via a coupling condenser I9 to the con-v 20 trol; gridcf pentodefllllibeng thus fsuperposed upon the. speech lyoltasepndrthe 4bias Yvoltages from,batteries- Silland 2|.;i tmaygloe:assumed,= for convenience, that the voltage of battery 4 5is iusnequsl .to the-reak -.v0ltase..0f. the loudest 251 spsechlwavssarrivng over transforman f Thus the tip of plug 3,35A is positivevvith respectto the sleeveV thereofl at allutirnes excepting A cnjtheextreme..inesativewpeaks ofthe speech-Waves lat which time it just reaches zerofpotentialwith 30:; respectto the sleeve), Foiconvenience, it may be further assumed that thevpotential `of, battery 2l is plsrsed'. ,bias jthe, eorlirelsridapsilively melted 0f negatively' and. sllfiellily large that when this Yollaseisslipsrpessd drenthe-voltage 35; of curve V,tlie most negative DQrifon Q f4 511 @UWG l V extends only` slightly below y@Zilli-riff. if this nisi so. andif it be assumed that atthe.momentv under considerationthe speech signal arflf'` in'gffrorn tran sfcrr'ner A32 has its niaxiniuinneg'atlve' value, s. imvi then' 'the resulting fwave applied to thecontrol grid fof pentode I4 willsb positioned asfshown in curve vW 'of' Fig; 2 Withitsmostj negative' por-l tion only' slightlyV below 'the cutoi threshold" E561? As previously described, thefscreenfl voltagel (and '45'H` perhaps`4 also the plate voltage) of 'the Vpentode A' I4 is' ofV low 'value' 'so' thatl the` cutoifthreshold Eco is'only a very few volts below the' gri'dcurrent thresholdEgc; Accordinglmas 'canf be'seen` Y from curve W of'FigjvZ, mostJ of` the voltage vvave` 50r` applied tothe control grid willllie above this grid currentthreshold; n n' Considering now in greater detail Athe actions-off the pentodeinresponseto the applied Avva-veW* shown inFig; 2, .it Will be seenfthatV yvvllerrthis 55 wavestarts upward from-its lowestfvalue below Eco there is at rst no responseluntil thewavef passes abovethe threshold EcmV Then as thewave continues ,to rise from .Ew toWardEgcan ampli;- fled. voltage Wave -is 4producedfin the outputfcir- 150V,l cuit of the pentodethe formpf thisvoltagewave -1 being shown in curve XcofwFig.. 2.1. (Actuallythis output ,voltagekwaveis of oppcsiteisignrfrom the ,input Wave, but to facilitate comparisonof the curvesit is shown asiftherewere noj'reversal 65' ofsigrrl When the. input `voltage.wave Vv'fpasseiv abovecnthe :upper threshold..ofaEgc` the outputx voltage( of the pentode abruptly ceases tof'varv by Lvir'tue of the ,follow/ing: The commencement; ovfggrid current flow. allscsagvoltage ,dropztoigm appear acrossresistanceg[Emilien subtract-sdram; the driving grid voltage thus effectively-vlimitingvvl the ygrid Swirls-ailles, polnif If the drivinssource '2 is' lof. f subslellllellnllsli .impsdaace,- -grid @sur-1V feuille-Wien@ to impose hier lafles lowers. ist:

ioggurve W, as

load the driving stage thus aiding this limiting action.

The'output voltage Wave X from the pentode I4 will therefore correspond exactly to those portions of curve W which lie between Eco and Een as can be plainly seen by comparing curves Wand Xsinf'Figa 2..y To Efa'cilitate comparison of theseland ,the .other curves;.the.curves-X, Y, Z are all shown with opposite polarity from the indicated by the designations 0-{72v at, the right of curve Z. Also, these three curves have their zero line offset with ref spectttd the-sacro linegof curve W to further facilitateicomparisonof curves X and Y. In adm Curve U of Fig 3. The prelmnarylipping ,1151 ditiomfitfwilllloemoted that the curves are drawn as if no ampl-ilcation occurred in any of the stages althgugli itis Aclear that amplification Will inherently occurin the clipper stage and may be included in other stages.

The output Wave X is next differentiated in ne'WQl 3510; Yfll Le series- 0f alternately positiveian'd: negative pulses asshown in `curve-X 'of i Fig'.42. The upward (ii le. negative) pulses corresponding', to; .tha inclined leading edgesl of `the m9311155isluaewalfxzllivef@ duration theoretically'c correspondir-lg togthe 4duration of--Ethese in- Glle@- Sllllef @Siwa-te :While the d0Wn-. Ward,.pulses?producedw .-byry thee-.vertical fftrailing edgespfthc-nearly sellar; Waves Xjhavetheoreticabilllyfsvgreater arntglitme Eandzerovfduration'.` These ,diilerencesffwhicm are-illustrated on f an; sxssseraieescv ell-nieuwer mswefsstuanr H1flle-Vel`lf:-H1iellie =byi prporionfina the :circuit Sothat theffgridgbase lire, thesvoltage diierbetweenA the threslicldelictangl EgcL is negli-` gibly --srnall ,inpconiparison fwith'q the height ofY l thsajvetootli L-\ lva.ve-l]- as rneasuredatithe 'con-x c trol 'grid'offthepen-tode 14.12,v It-will beiinOted that the; c ornplete-:sayv-tooth wavegformfU is never aqiliellr-applissi-.io ihefgridofspemode; M .but` faillertonllaihelle c rl-portion is-amp1iedcand.; appliedto VY.the grid las .Wave If; however, thev wai/elw be teCQnstrucicd-by extrapolation S0 tod@termine-thereinplitude,ofgthefcorresponding 1 complellsavvetooth Walleat the ,input offpentode. i lflffland sii-theacomplete height ,'fromrpeakL-to valley o ,this reconstructed :saw-tooth, `yvave.,` bef designated as1 l?, thengthe iffg-rid ibasel. orv voltages interval `ifronri Ec"o i;to-='Ege. is ',desir'ablyqless'` than four percentandpreferably'iless than twopercent oil??A Infthe, curveschen,.fcrillustration; since the wave-,fil isucut inl'half to; produce. .the .wave theqi'leighti-:of thefcblanked, sawtooth-lf-wave actually applied to the grid of pentode i4 is l 2 and thefgrid base :is eiglitpe rl-:entl o fqthis actual. height;

2 :i bln'gtnasjfpur, percent Aof "the height. vi "ory the. imaginai'y,A complete ksa'Wc-tootl'l wave correspond- *l ingi lto Lthe" actual'.blankedsawetoothgwaye applied to the' inpriifof,K'pentodel,il4.,By suchjproportlqnirialthe' width-,ofgthsj upward pulses shown m urvev'@maissxsedlnelrsmau in-compsrisla: @che lsrrsnr interval entdftherefore the: difieren btufeenfthewdthsfof ,the npwardland i dogr-nyvardgpulfses frepresentedfinfourve .Y is small alri i will to -a considerable; extent,- :bev maskedsby t the inevitable rounding and broadening ;of;pulsesA which;occursy in ,transmitting vthem tl'lrcugli` any practical circuit. ;,Itgwillbelgunderstooll; Athere-.

fore, that the representation of curve Y which shows the positive and negative pulses of substantially different form, is merely intended to clarify the principles involved and is not intended to indicate that any very great difference in width may be apparent. The alternate positive and negative pulses from network 35 are then transmitted through a network comprising a conventional full wave rectier and a limiter device preferably including amplification. After such full Wave rectification and limiting, the pulses will have a similar polarity and amplitude, as can be seen in curve Z of Fig. 2.

Thevabove description of operation, as well as the associated curves W, X, Y, Z of Fig. 2 relates to the operation during the intervals when .the speech wave or other modulating signal has substantially its maximum negative value (so that the potential of the speech wave just offsets the potential of battery 34, giving the tip of plug 33 a zero potential withI respect to its sleeve). In contrast to the conditions occurring during such time it will be useful to consider the conditions occurring during the opposite extreme of the modulating signal (i. e., when the tip of plug 33 has its maximum positive potential with respect to the sleeve thereof). Operation during the latter condition is illustrated in curves W', X', Y', Z in the lower part of Fig. 2, these curves being aligned with the corresponding curves W, X, Y, Z for ease in comparison.

The operations of generator I and preliminary clipping amplifier I2 are unchanged regardless of the potential of the modulatingv signal, and consequently the curves U and V apply just as before. Because of the positive potential of .the plug tip withrespect to its sleeve the control grid of pentode I4 is biased much less positively than before and therefore the applied blanked saw-tooth waves W' are positioned lower with respect to the cut-olf level Eco. In the case chosen for illustration it is assumed that the speech signal voltage added to the voltage of battery 34 gives a resultant negative bias large enough to overcome the positive bias of battery 2| and thus to bring the main control grid potential below zero. Accordingly, curve W is illustrated as lying predominantly below the threshold Ew. Actually it is usually preferable to employ a smaller modulating signal so that the grid bias would become a little less positive only and the curve W would be positioned a little lower than curve W.

The clipping action of the circuit I3 takes place essentially as before thus producing the wave X' in the same manner previously described for the wave X. .Because of the different positioning of the curve W' with respect to the two thresholds Eco and Ege, however, the generallyv rectangular prominences of wave X are Vsomewhat narrower than the corresponding ones of wave X.y The differentiating network 35 produces curve Y with alternate upward and downward impulses in the same manner as described for curve Y, but owing to the narrower prominences of curve X the time interval between each upward impulse and the succeeding one is substantially smaller than for the curve Y. The full wave rectication and clipping action of circuit 36 then brings all of the pulses to similar amplitudes and polarity as illustrated in curve Z' thus producing pairs of pulses whose spacing is somewhat closer than the spacing of the corresponding pairs in curve Z.

6. been produced as described they may be directly transmitted over a wire line or otherwise for communication or other purposes. In the preferred embodiment of our invention these pulses are applied to modulate the radio frequency amplifier 33, thus yielding brief radio frequency wave trains, hereinafter referred to as carrier pulses. These carrier pulses are then radiated over antenna 39.

It may be that on a purely theoretical ba-sis the inclusion of the preliminary clipping amplifier I2 wouldnot appear essential, since the saw-tooth waves U could theoretically be directly applied to the clipping circuit I3 in place of the blanked waves V. Inclusion of the preliminary clipping amplifier I2, however, is regarded as one of the useful novel features of our invention, since it greatly facilitates the design of equipment for performing the very narrow double clipping action shown and described.

As previously mentioned, it is desirable to minlmize the widths of the upward (i. e. negative) pulses of Y and Y by proportioning the biases and wave amplitudes so that the grid base (i. e., the number of volts between the cut-0E threshold Eco and the grid current threshold Ego) is very many times smaller than the theoretical amplitude of the complete saw-tooth wave corresponding to the blanked saw-tooth at the input of the main clipping circuit. If no preliminary clipping amplifier I2 were provided and if it were desired that the grid base should be less than say 2% of the peak-to-peak voltage swing of the complete saw-tooth wave,\then it would be necessary to apply to the input of the main clipping circuit a full saw-tooth wave having fifty times the voltage of the grid base. To apply such a large voltage to the input grid of the pentode might require the use of an unnecessarily large tube at this stage; and furthermore the generation of such a large amplitude saw-tooth might require unnecessarily large tubes and large powers for the preceding stages. In accordance with the present invention, however, only a small portion of the whole wave need be employed for transmission through the main clipping circuit since even with the maximum bias shifts caused by the input speech amplifier the position-s of the thresholds Ecc and Egc never shift below the upper half of the wave. The provision of the preliminary clipping amplifier I2 therefore enables the reduction of the power handling capacity and size of the vequipment while still enabling the maintenance of a very small ratio between the grid base and the theoretical peak-to-peak voltage swing and the imaginary wave corresponding to the actual input wave of the clipping circuit.

In a preferred form of the invention the speech waves have still smaller amplitude than represented by the curves'of Fig. 2 so that the curve W is only slightly lower than the curve W with respect to the clipping thresholds. It is therefo're possible and desirable in accordance with the preferred form of the invention to arrange the clipping amplifier I2 so as to cut off not only the lower half of the saw-tooth wave V but also the upper third thereof thus leaving only one-sixth Aof the output wave for transmission to the main clipping circuit I3. In the curves chosen for illustration, however, the voltage swing is taken as several times greater than it would actually be in order that the differences between the waves X and X or between waves Z and Z may be more readily visible in the drawings.

In the above described embodiment of the in- 9 to the output portion of the equipment vcompris-- ing elements |35, |36, |31, |38 and |39 which correspond to the similarly numbered elements of Fig. 1.

'Ihe operation of the system shown in Fig. 6 can best be understood by consideration of this figure in conjunction with the curves of Fig. 7. Sine waves from source pass through amplier and preliminary clipping amplifier ||2 and are thus converted into the approximately trapezoidal form shown in curves A and A of Fig. 7. In the curves of Fig. 7 it is assumed that the upper and lower quarter of the sine wave have been removed by the clipping amplifier ||2 so that only the central half remains. Since it is not essential that the clipping action of amplifier I2 be a sharp clipping action giving perfectly square corners the curves A and A of Fig. 7 have been shown with slightly rounded corners.

The curve A represents conditions when the speech input wave has its maximum negative value so that the total voltage produced by the speech wave and battery together is zero. At this time the main clipping circuit acts just as if the plug |33 were not plugged in. The input wave A is symmetrical with respect to thezero line as shown so that the potential at the lefthand side of the rectier I4 varies equally above and below ground potential. Because of the rectifying action of element ||4 the intermediate point M of the clipping circuit is unable to be raised above its natural or open circuit potential since it cannot receive current through rectier ||4. Thus the potential of point M will follow the input potential only up to the point where the input potential equals the potential of battery H9 plus the voltage drop across RIZU. Any potential rise beyond this limit (represented in Fig. 7 by a dotted line marked upper clipping limit) will not be transmitted to point M so that only those portions of curve A lying below such upper clipping limit will pass through to point M. In transmission from point M to the output of the main clipping circuit the rectifyingV action of element similarly eliminates all potential variations below ground potential, since the point M cannot draw current from the right-hand side of the circuit but can only deliver current thereto. As a result all portions of the curve below the lower clipping limit (i. e., below the zero line) will be blocked and not transmitted to resistor |23.

Therefore, the only parts of curve A which will be transmitted to resistor |23 will be those portions between the two dotted lines marked, respectively, upper clipping limit and lower clipping limit of Fig. 7. The resulting truncated trapezoidal waves are shown in curve B of Fig. 7. (Actually the corners of such waves will not be perfectly square, but since it is desirable to have the clipping as sharp as practicable the corners are shown in the ideal square form.)

As in the previously described system the clipped waves are next transmitted to difierentiating network |35 in which they are converted into alternately positive and negative pulses whose time duration corresponds to the time duration of the sloping sides of the trapezoidal waves of curve B, The waves produced by such differentiation are illustrated in curve C of Fig. 7. The pulses of curve C are desirably less than one-ftieth of the total cycle interval and preferably less than one per cent. of such interval. Accordingly, the voltage separation between the upper and lower clipping limits associated with curve A should be less than four per cent.,ofthe 1,0 peak-to-Valley voltage swing of the imaginary complete sine wave corresponding to the. truncated wave Aapplied to the main clippingcircuit 3, and preferably should be less than two per cent. of this peak-to-valley voltage swing. For facility of illustration, however, the separation between the upper and lower clipping limits is shown somewhat greater than six per cent. of the whole voltage swing, and the duration of the sloping sides of curve B and the pulses of curve C are greater than would be desirable in practice.

Y negative bias is thereby applied through low resistor |22 to the left side of rectifier ||4 and thus the truncated sinusoidal input from amplifier I I2 is'superposed on this negative bias so as the resulting wave A' lies principally below zero potential, as shown in the lower portion of Fig. 7. Ac-

cordingly, the trapezoidal wave form B-' produced by the clipping action is correspondingly narrow; and the corresponding pairs of pulses of C are closer together, v

The pulse output from the diierentiating network |35, whether it has the form of curve C or that of curve C', or some intermediate form. is then applied to the full wave rectiiying and-limiting circuit |36 which renders the pulses unidirectional and equal as described in connection with Figs. 1 and 2. These rectified pulses are then used to modulate amplifier |38, which is excited by source |31; and the brief carrier pulses thus Produced are radiated over antenna |39. Al-

v though clipping circuit ||3 has been shown in connection with the system in Fig. 6, it will be understood that any one of the three clipping circuits i3, I3' or ||3 may be used in either the system of Fig. 6 or that of Fig. 1 (or in the system of Fig. 1 as modified by replacing I0 and with the circuit of Fig. 4). It should also be understood that each Aof these circuits may be further replaced by its equivalents, for example by substituting electronic diode rectiiiers for the dry -rectiflers ||4 and 5 in circuit I3, or by substituting triodes for the pentodes in circuits i3 and I3. In fact the system at present believedpreferable corresponds to Fig. 6 with electronic diodes substituted for the dry rectiers illustrated. l

Instead of plugging the speech modulating waves into jack I6 of Fig. 6, the plug may be inserted into jack |09 of the amplifier |||l Such amplier may then be arranged to modulate the wave in any known manner such as by amplitude modulation, phase modulation, or frequency modulation. Preferably, however, amplitude modulation is employed since this will'result in the symmetric time displacement of alternate pulses, which is at present considered to be the preferred form of time modulation. If amplitude modulation of the sine waves is effected in amplifier ||0, it is then preferable that the incoming wave should be related to the clipping limits in the manner of wave A' rather than in the manner of wave A.' Accordingly, battery |40 and switch |4| are provided to enable a negative bias to be applied via resistor |22 when required.

ulating signals of a lower frequency than said l0 foundation Waves, and means under the control of said modulating signals for varying simultaneously the relative relationship 0f both said limits with respect to said xed reference axis of said foundation waves While maintaining said axis l5 at its fixed Value.

EMILE lLABIN. DONALD D. GRIEG.

REFERENCES CITED Tfhe following references are of record in the 5 ille of this patent:

UNITED STATES PATENTS Number Name Date HelsingV Jan. 10, 1943 Kell Nov. 24, 1936 Luck July 13, 1937 Luck Apr. 5, 1938 Luck Jan. 7, 1941 Kell Apr. 21, 1942 Foster et al. Dec. 16, 1941 Landon Jan. 18, 1938 Hugon July 16, 1940 

